Re: [PATCH v4 02/12] virtiofsd: Limit setxattr()'s creds-dropped region

[Hanna Reitz]

On 18.10.21 19:20, Vivek Goyal wrote:
On Thu, Sep 16, 2021 at 10:40:35AM +0200, Hanna Reitz wrote:
We only need to drop/switch our credentials for the (f)setxattr() call
alone, not for the openat() or fchdir() around it.
(Right now, this may not be that big of a problem, but with inodes being
identified by file handles instead of an O_PATH fd, we will need
open_by_handle_at() calls here, which is really fickle when it comes to
credentials being dropped.)

Signed-off-by: Hanna Reitz <hre...@redhat.com>
---
  tools/virtiofsd/passthrough_ll.c | 34 +++++++++++++++++++++++---------
  1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 6511a6acb4..b43afdfbd3 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -3123,6 +3123,7 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, 
const char *in_name,
      bool switched_creds = false;
      bool cap_fsetid_dropped = false;
      struct lo_cred old = {};
+    bool changed_cwd = false;
  
      if (block_xattr(lo, in_name)) {
          fuse_reply_err(req, EOPNOTSUPP);
@@ -3158,6 +3159,24 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, 
const char *in_name,
               ", name=%s value=%s size=%zd)\n", ino, name, value, size);
  
      sprintf(procname, "%i", inode->fd);
+    /*
+     * We can only open regular files or directories.  If the inode is
+     * something else, we have to enter /proc/self/fd and use
+     * setxattr() on the link's filename there.
+     */
+    if (S_ISREG(inode->filetype) || S_ISDIR(inode->filetype)) {
+        fd = openat(lo->proc_self_fd, procname, O_RDONLY);
+        if (fd < 0) {
+            saverr = errno;
+            goto out;
+        }
+    } else {
+        /* fchdir should not fail here */
+        FCHDIR_NOFAIL(lo->proc_self_fd);
+        /* Set flag so the clean-up path will chdir back */
+        changed_cwd = true;
Is there a need to move FCHDIR_NOFAIL() call earlier too? I am assuming
this will not be impacted by file handle stuff. So we probably could
leave it in place. Easier to read.

I wanted to limit the region where the creds are dropped to an absolute 
minimum, i.e. just around (f)setxattr().  I prefer this in general, not 
just because it breaks opening file handles, and so I wanted to pull out 
not just the openat(), but the fchdir() as well.

Hanna